The present invention relates generally to a powder coating system and more particularly to a distributed control system providing a gun control for each spray gun which selects a particular one of a plurality of stored sets of powder dispensing parameters and independently controls the triggering of its powder spray gun.
A powder coating system sprays an electrostatically charged airborne powder within an enclosure or booth containing the part or article to be coated. The electrostatic potential between the powder and the article causes the powder to be attracted to and move into contact with the surface of the article. The deposited powder is then heated so that it flows and hardens on the surface on which it has been deposited.
The present invention relates to two areas of powder spray control. First is the selection and control of certain spray parameters, for example, the powder flow air pressure, the atomizing air pressure and pattern air pressure, if required. In addition, with corona type spray guns, an electrostatic voltage is selected and supplied by an internal power supply. The second area of powder spray control is gun triggering, that is, when the spray gun is turned ON and OFF, in relation to parts traveling through the spray booth. In the most basic systems, the air pressures and electrostatic voltage are controlled by manually setting respective pressure regulators and a power supply, and the gun triggering is also manually controlled.
Some systems have been developed that automate the gun triggering. For example, the xe2x80x9cSMART SPRAY(copyright)xe2x80x9d gun controller which is manufactured and sold by Nordson Corporation of Amherst, Ohio, the assignee of the present invention, uses a microprocessor based gun controller in combination with manually set pressure regulators to automatically control the spray gun triggering. The gun controller operates with photodetectors in the spray booth to provide gun triggering in different spray booth zones. Either a conveyor feedback transducer or control timer is used with the photodetectors to detect the presence of a part as well as its front and rear edges as it travels through the booth, and the gun controller triggers the gun ON and OFF in response to the photodetectors sensing part presence. However, the spray parameters remain constant unless they are manually changed by the operator.
In other systems, a programmable logic controller (xe2x80x9cPLCxe2x80x9d) is used as a centralized powder spray system control in association with photodetectors and a conveyor feedback transducer. The photodetectors and a feedback transducer from the conveyor sense the presence and identity of different parts, respectively to be coated as well as line gaps between successive parts on the conveyor. The PLC can be operatively connected to voltage to pressure transducers for selecting the desired powder air flow, atomizing air and pattern air pressures. The centralized PLC turns selected spray guns ON or OFF as a function of the part identified and line gaps between parts.
While the above systems have performed satisfactorily, they utilize a centralized controller or PLC which singularly controls the triggering of each of the spray guns, and further, singularly controls each of pressure regulators and each of the power supplies for each of the guns. This centralized system control configuration has a disadvantage of requiring extensive wiring within the painting facility much of which must be done upon installation at the user""s site. Moreover, a PLC is not adept at performing complex arithmetic operations and handling more complex data structures. PLC""s have the further disadvantage of only providing a limited amount of process status information to the operator or other analytical devices. Furthermore, the use of a PLC as the centralized control system has a further disadvantage in that it is difficult and expensive to change the electrical configuration of the control system. Also, there is no redundancy in a centralized PLC control system and any electrical failure within the PLC will terminate the operation of the entire coating system.
Still further, because the single centralized PLC must serially process data for each of the powder dispensers, there is a further disadvantage in that the processing bandwith, that is, the real time window in which the PLC can dedicate to processing data for a particular powder spray gun is relatively small. Therefore, more comprehensive control of the powder spray cycle is very difficult. For example, with a centralized control, a gun purge cycle to clean the dispensing hose and spray gun is not programmable. When the spray gun is triggered ON powder is pumped from the powder source, through a dispensing hose up to thirty feet long and then through the spray gun. When the spray gun is triggered OFF, the fluidizing air pressure in the dispensing hose is terminated; and therefore, the powder in the dispensing hose separates from its transport air and often settles and collects in lumps or clumps in the hose. When the spray gun is again triggered ON, the powder lumps are sprayed in an uneven manner. With the prior control systems, a gun purge cycle is manually controlled by the operator when it is required.
Finally, as the system size, in terms of the number of powder dispensers and spray guns increases, the added complexities of using a single centralized PLC cause its costs to increase substantially.
To overcome the disadvantages described above and to provide a highly flexible control system with capabilities not previously found in centralized powder coating control systems, the present invention eliminates the centralized control of all of the spray gun functions and provides a powder coating control system wherein control is distributed in a new and more efficient manner, thereby minimizing of wiring within the powder spray booth. The triggering and selection of spray parameters of each of the powder spray guns within the control system of the present invention is independently and individually controlled by its own gun control so that each gun is capable of more comprehensive powder coating process control. Therefore, the control system of the present invention has greater flexibility and reliability with less complex wiring. The control system of the present invention is particularly beneficial in being able to select different sets of powder dispensing parameters on-line and in real time to make the powder coating process more efficient and cost effective.
According to the principles of the present invention and in accordance with the described embodiments, a powder coating system includes a plurality of powder spray guns disposed with respect to an article to be coated. Each of the powder spray guns is connected to its own gun control which stores a set of spray parameters and triggers its spray gun ON and OFF to apply a powder coating in accordance with the stored spray parameters. A communications network is in electrical communication with the plurality of gun controls. Providing a control for each powder spray gun results in a control system that is modular, highly flexible and provides a more comprehensive powder coating process control. A dedicated control for each powder spray gun has the advantage of being able to report more process status information to the operator control, thereby permitting more comprehensive statistical process control as well as more sophisticated automatic diagnostic procedures. The communications network advantageously simplifies the wiring between control components within the coating system, thereby reducing the cost of installation. With multiple controls, a failure of one control does not necessarily require the powder coating operation be completely shut down thereby providing further advantages in efficiency and cost savings.
In a further embodiment of the invention the powder coating system includes a sensor responsive to a conveyor moving the part past the spray gun which can be used to provide system signals representing first, a change in the position of the part, and second, a physical characteristic of the part. Therefore, the spray parameters may be changed in real time as one or more parts or portions of parts are moved through the spray booth.
In another embodiment, each of the gun controls in the powder coating system includes a network interface, a memory for storing sets of spray parameters, a digital to analog converter and a processor for triggering its respective spray gun ON and OFF to apply a powder coating in accordance with the stored set of spray parameters. In a still further embodiment, the powder system control includes a system control connected to the communications network for providing data to and receiving data from the gun controls.
In another aspect, the invention includes a method of applying a powder coating on a part moving with respect to powder spray guns by storing a plurality of presets of spray parameters in each of a number of gun controls connected to a like number of powder spray guns. The part to be coated is detected and the appropriate gun controls are activated to select a preset of gun operating parameters as a function of detecting the part. The above method can be implemented by each of the gun controls selecting different presets of spray parameters in response to detecting different physical characteristics of one or more of the parts or portions of the parts. In a further aspect of the above method, the different presets of spray parameters are detected in response to detecting changes of position and different physical characteristics of the one or more parts moving with respect to the powder spray guns.
In another embodiment of the invention, a gun purge cycle is programmable and automatically executed as part of a standard powder spraying process. With a tribo gun, in which the electrostatic charge is created by the static electricity of the powder flowing through the spray gun, it has been found that purging is desirable prior to the execution of a powder spraying process. With the present invention, a purge-on cycle may be programmed to automatically purge only the spray gun after the part has been detected but prior to the part arriving at the spray gun. Further, at the end of a powder spraying process, a purge-off cycle may be programmed to use pressurized air to automatically clean the powder dispensing hose and the spray gun of excess powder. Consequently, the invention provides an automatic powder spray cycle that prevents the surging and spitting of undispensed powder at the start of powder dispensing cycles. Therefore, another advantage of the invention is that, for the first time, a powder dispensing process can be programmed that changes powder spray parameters in real time.
In a still further embodiment, the invention includes a method of operating a powder coating system in which the plurality of gun controls are automatically initialized and brought on-line in a fully operable state without any operator intervention. The control system has the capability of detecting when one gun control is replaced by another, or when a new gun control is added to the system. Consequently, the method provides a significant savings in system downtime and operator time that would be otherwise required to initialize the gun controls.